A conventional X-ray diagnostic imaging apparatus for angiography or tomography has a function for changing a visual field size displayed on a monitor screen. For changing the visual field size of a displayed image on the monitor screen, two types of methods has been applied. One is to change the detection field size of an image intensifier (hereinafter called as an I.I.), and the other is to change the magnification ratio of the digital data of the collected image.
Usually, for changing the visual field size, the detection field size of the I.I. is mainly adjusted. To change the visual field size, an operator selects one switch among a plurality of switches, each corresponding to a respective detection field size. Thus, when the detection field size of the I.I. is adjusted, the visual field size is changed. However, due to the character of the I.I., it difficult to gradually change the detection field size. Accordingly, it was able to choose only three or four levels. In order to overcome this disadvantage, a method of changing a magnification ratio of digital image data has been introduced. When an operator inputs a value of a particular magnification ratio, the visual field size is automatically adjusted. Since the magnification ratio is digitally changed, the visual field size can be gradually adjusted. However, when the magnification ratio was increased, the image per pixel has been slightly out of focus. Accordingly, the conventional digital method is practically limited to use for only a narrow range. The conventional X-ray diagnostic imaging apparatus can change the visual field size gradually with appropriate focus, by using both methods.
In addition, when the detection field size of the I.I. becomes small, an amount of the incidence X-ray per pixel on a camera attached to the I.I. decreases. Hence, an image defect due to noise is generated on the monitor screen, which makes it difficult for an operator to observe the image.
In order to solve this problem, it has been proposed to automatically change the X-ray condition according to the detection field size of the I.I. That is, the X-ray condition is automatically set in a high position, when the detection field size of I.I. is changed to be a small. However, such a conventional X-ray diagnostic imaging apparatus includes further problems. Thus, in order to gradually change the visual field size, an operator has to select the switch to set the detection field size of the I.I. and input the magnification ratio, respectively. Further, the operator has to calculate the appropriate magnification ratio, considering the detection size of the I.I. in order to set the desired visual field size, since both operations are independent. Consequently, it takes much time for an operator to perform these operations. Moreover, in order to reduce the x-ray to the patient, the X-ray irradiation must be stopped while the operator is adjusting the visual field size.
In addition, when the magnification ratio is increased, an amount of the incident X-ray per pixel on the camera is decreased. Consequently, noise is generated on the monitor screen, which makes it difficult for the operator to observe the image. With the conventional X-ray diagnostic imaging apparatus, the X-ray condition is automatically changed according to the detection field size of I.I., but it is not changed according to the magnification ratio of the digital data. For example, the detection field size of the I.I. can be selected among three levels, such as 9 inches, 7 inches and 5 inches, and the visual field size can be changed by 0.5 inch from 4.5 inches to 9.0 inches, by adjusting the magnification ratio. As illustrated in FIG. 1, the X-ray condition keeps the constant A when the visual field size is between 7.0 and 9.0 inches, it keeps the constant B that is higher than the constant A when it is between 5.5 and 7.0 inches, and it keeps the constant C that is higher than the constant B when it is between 4.5 and 5.0 inches. In this case, when the visual field size is 7.0, the detection field size of I.I. is 7.0 inches and the magnification ratio is 1.0 time, for example. While, when the visual field size is 5.5 inches, the visual field size is 5.0 inches and the magnification ratio is 1.27 times, for example. Thus, when the detection field size of I.I. is 7.0 inches and 5.0 inches, each of the X-ray conditions is the same condition B as long as each magnification ratio is different.
Therefore, the image clearly has more noise in the latter case and becomes illegible.